1. Field of the Invention
The present invention relates to a method of treating electrically conductive waste contaminated with nuclear fuel materials and disposed from a nuclear fuel handling facility, and an apparatus for carrying out the method. More specifically, the present invention relates to a method of treating contaminated metallic waste produced when a nuclear fuel handling facility is dismantled, such as waste steel materials contaminated with nuclear fuel materials, or an adsorbent used for adsorbing nuclear fuel materials mounted in a nuclear fuel handling facility, and an apparatus for carrying out the method.
2. Description of the Related Art
FIG. 20 is a typical view of an apparatus for carrying out an electrolytic polishing process generally used for decontaminating waste contaminated with radioactive substances, such as nuclear fuel materials, (hereinafter referred to as xe2x80x9cradioactive contaminated wastexe2x80x9d) by electrolysis. As shown in FIG. 20, a radioactive contaminated waste 3 is held by a holding device 2 and is immersed in an electrolytic solution contained in an electrolytic vessel 1 of a stainless steel. The radioactive contaminated waste 3 functions as an anode. A cathode 4 is immersed in the electrolytic water solution 5. When the radioactive contaminated waste 3 is a stainless steel waste, a phosphoric acid solution is used as the electrolytic water solution 5 serving as a bath. When the radioactive contaminated waste 3 is a carbon steel material, a sulfuric acid solution is used as the electrolytic water solution 5. The holding device 2 and the cathode 4 are connected to a dc power supply 6. When a dc voltage is applied across the holding device 2 and the cathode 4 by the dc power supply 6, the radioactive contaminated waste 3 functions as an anode. A surface layer of the radioactive contaminated waste 3 dissolves in the electrolytic water solution 5 simultaneously with coming off of radioactive contaminants adhering to the radioactive contaminated waste 3. Part of substances came off the radioactive contaminated waste 3 remains in the electrolytic water solution 5 and the rest is precipitated in sludge 8 on the bottom of the electrolytic vessel 1. Hydrogen 7 is produced on the cathode 4 of a stainless steel.
Generally, when decontaminating a radioactive contaminated waste by the electrolytic polishing process using the electrolytic water solution 5 as a bath, current is unable to flow uniformly over the entire surface of the radioactive contaminated waste when the radioactive contaminated waste has a complicated shape because the resistance of the bath is high. Consequently, the decontaminating effect of the electrolytic polishing process is reduced for some portions of the radioactive contaminated waste. If a high current is supplied to the bath to enhance electrolytic processing speed, heat is generated in the bath due to the high resistance of the bath. Hydrogen 7 produced on the cathode 4 during the electrolytic polishing process cause problems in safety. It is difficult to remove radioactive substances accumulated in the waste electrolytic water solution 5, particularly, radioactive substances dissolved in the waste electrolytic water solution 5 from the waste electrolytic water solution 5. The waste electrolytic solution 5 cannot be reused and becomes an additional radioactive contaminated waste. Thus, the total amount of radioactive contaminated waste increases.
The present invention has been made to solve those problems and it is therefore an object of the present invention to provide a method of treating waste from a nuclear fuel handling facility, capable of easily decontaminating a contaminated waste having a complicated shape, not discharging any effluent, capable of repeatedly using an electrolytic solution and not producing additional waste.
Another object of the present invention is to provide an apparatus for carrying out the foregoing method.
According to a first aspect of the present invention, a method of treating electrically conductive waste contaminated with nuclear fuel materials from a nuclear fuel handling facility comprises a molten salt electrolysis process for removing the nuclear fuel materials adhering to a surface of the waste by immersing the waste in a molten salt to dissolve a surface layer of the waste electrochemically in the molten salt; and a filtering process for filtering the molten salt used in the molten salt electrolysis process to extract the nuclear fuel materials removed from the surface of the waste and accumulated in the molten salt from the molten salt. The molten salt filtered in the filtering process is reused in the molten salt electrolysis process.
Preferably, the method further comprises an evaporation process for removing the molten salt adhering to a surface of the waste processed by the molten salt electrolysis process and taken out of the molten salt by heating the waste so that the molten salt adhering thereto evaporates. The molten salt recovered in the evaporation process is reused in the molten salt electrolysis process.
Preferably, the method further comprises a cleaning process for removing the molten salt adhering to the waste processed by the molten salt electrolysis process and taken out of the molten salt by a cleaning liquid, and an evaporative drying process for drying the molten salt contained in the cleaning liquid by evaporating the cleaning liquid used in the cleaning process. The molten salt recovered in the evaporative drying process is reused in the molten salt electrolysis process, and the cleaning liquid evaporated in the evaporative drying process is reused in the cleaning process.
Preferably, in the molten salt electrolysis process, the molten salt and the waste immersed in the molten salt are moved relative to each other to remove the nuclear fuel materials from the surface of the waste.
Preferably, in the molten salt electrolysis process, the waste is contained in a basket serving as an electrode for an electrolysis and the basket is vibrated in the molten salt.
Preferably, in the molten salt electrolysis process, the waste is contained in a basket serving as an electrode for an electrolysis and the basket is rotated in the molten salt.
Preferably, in the molten salt electrolysis process, the molten metal is spouted against the waste immersed in the molten salt.
Preferably, a liquid metal, which is in a liquid phase at a temperature high enough to maintain the molten salt in a molten state, is placed in the molten salt as an electrode for the molten salt electrolysis process.
Preferably, when the nuclear fuel materials are oxides, the method further comprises a reducing process for reducing the nuclear fuel materials to metals before subjecting the waste to the molten salt electrolysis process.
Preferably, in the reducing process, the nuclear fuel materials are reduced to metals by making the nuclear fuel materials react with a reducing agent.
Preferably, the reducing process comprises immersing the waste contaminated with the nuclear fuel materials in a reducing molten salt, supplying a reducing agent into the reducing molten salt, applying a voltage that will not cause a decomposition of the reducing molten salt across an anode and a cathode immersed in the reducing molten salt to regenerate the reducing agent reacted with the nuclear fuel materials.
Preferably, the reducing process comprises immersing the waste contaminated with the nuclear fuel oxides in a reducing molten salt, reducing the nuclear fuel oxides to metals by applying a voltage across an anode and a cathode immersed in the reducing molten salt for an electrolytic reduction.
According to a second aspect of the present invention, a method of treating an electrically conductive waste contaminated with nuclear fuel materials from a nuclear fuel handling facility comprises a reducing process for reducing the nuclear fuel materials to metals; a thermal melting process for producing a molten salt by heating and melting the metals produced by reducing the nuclear fuel materials and the waste; and a molten salt electrolysis process for recovering the metals produced by reducing the nuclear fuel materials and contained in the molten salt by applying a voltage across an anode and a cathode immersed in the molten salt so that the metals produced by reducing the nuclear fuel materials are deposited on the cathode.
Preferably, a chloride or a hydride having a same kind of cation as that of the molten salt is added to the molten salt to lower the melting point of the molten salt so that an operating temperature of the molten salt in the molten salt electrolysis process is lowered.
Preferably, the method further comprises a cleaning process for separating the nuclear fuel materials from the waste by cleaning the nuclear fuel materials deposited on the cathode in the molten salt electrolysis process and the waste with a cleaning liquid to dissolve the waste in the cleaning liquid; and an oxidation process for converting the nuclear fuel materials separated from the waste by the cleaning process into oxides by oxidizing the nuclear fuel materials; wherein the waste is an adsorbent used in the nuclear fuel material handling facility.
Preferably, the method further comprises an evaporative drying process for drying the adsorbent contained in the cleaning liquid by evaporating the cleaning liquid used in the cleaning process. The cleaning liquid evaporated by the evaporative drying process is reused in the cleaning process.
According to a third aspect of the present invention, an apparatus for treating an electrically conductive waste contaminated with nuclear fuel materials from a nuclear fuel handling facility comprises a molten salt electrolysis unit for removing the nuclear fuel materials adhering to a surface of the waste by immersing the waste in a molten salt to dissolve a surface layer of the waste electrochemically in the molten salt; a filtering unit for filtering the molten salt used by the molten salt electrolysis unit to extract the nuclear fuel materials removed from the surface of the waste and accumulated in the molten salt from the molten salt, and a molten salt return line for returning the molten salt filtered by the filtering unit to the molten salt electrolysis unit.
Preferably, the apparatus further comprises an evaporation unit for removing the molten salt adhering to a surface of the waste processed by the molten salt electrolysis unit and taken out of the molten salt by heating the waste so that the molten salt adhering thereto evaporates, and a molten salt return line for returning the molten salt removed from the surface of the waste by the evaporation unit to the molten salt electrolysis unit.
Preferably, the apparatus further comprises a cleaning unit for removing the molten salt adhering to the waste processed by the molten salt electrolysis unit and taken out of the molten salt by a cleaning liquid, and an evaporative drying unit for drying the molten salt contained in the cleaning liquid by evaporating the cleaning liquid used by the cleaning unit, a molten salt return line for returning the molten salt recovered by the evaporative drying unit to the molten salt electrolysis unit, and a cleaning liquid return line for returning the cleaning liquid evaporated by the evaporative drying unit to the cleaning unit.
Preferably, the molten salt electrolysis unit is provided with a driving mechanism for moving the molten salt and the waste immersed in the molten salt relative to each other.
Preferably, the molten salt electrolysis unit is provided further with a basket capable of containing the waste and serving as an electrode for an electrolysis, and the driving mechanism vibrates the basket in the molten salt.
Preferably, the molten salt electrolysis unit is provided further with a basket capable of containing the waste and serving as an electrode for an electrolysis, and the driving mechanism rotates the basket in the molten salt.
Preferably, driving mechanism includes a spouting means for spouting the molten salt against the waste immersed in the molten salt.
Preferably, the molten salt electrolysis unit is provided with an electrode formed from a liquid metal which is immersed in the molten salt and is in a liquid phase at a temperature high enough to maintain the molten salt in a molten state.
Preferably, when the nuclear fuel materials are oxides, the apparatus further comprises a reducing unit for reducing the nuclear fuel materials to metals.
According to a fourth aspect of the present invention, an apparatus for treating an electrically conductive waste contaminated with nuclear fuel materials from a nuclear fuel handling facility comprises a reducing unit for reducing the nuclear fuel materials to metals, a thermal melting unit for producing a molten salt by heating and melting the metals produced by reducing the nuclear fuel materials and the waste, and a molten salt electrolysis unit for recovering the metals produced by reducing the nuclear fuel materials and contained in the molten salt by applying a voltage across an anode and a cathode immersed in the molten salt so that the metals produced by reducing the nuclear fuel materials are deposited on the cathode.
Preferably, the waste is an adsorbent used for adsorbing the nuclear fuel materials in the nuclear fuel handling facility, and the apparatus further comprises a cleaning unit for separating the nuclear fuel materials from the waste by cleaning the nuclear fuel materials deposited on the cathode of the molten salt electrolysis unit and the waste with a cleaning liquid to dissolve the waste in the cleaning liquid, and an oxidation unit for converting the nuclear fuel materials separated from the waste by the cleaning unit into oxides by oxidizing the nuclear fuel materials.
Preferably, the apparatus further comprises an evaporative drying unit for drying the adsorbent contained in the cleaning liquid by evaporating the cleaning liquid used by the cleaning unit, and a cleaning liquid return line for returning the cleaning liquid recovered by the evaporative drying unit to the cleaning unit.